Keep temporary directory in case of failure

[hugelgupf]

Updates u-root/u-root#2603
Updates u-root/u-root#2814

[codecov]

Codecov Report

Attention: Patch coverage is 35.71429% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 75.54%. Comparing base (cbbc9b1) to head (98f883d).

Files	Patch %	Lines
cmd/mkuimage/main.go	50.00%	2 Missing and 3 partials ⚠️
uimage/builder/gbb.go	0.00%	3 Missing ⚠️
uimage/uimage.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   75.62%   75.54%   -0.08%     
==========================================
  Files          21       21              
  Lines        1366     1370       +4     
==========================================
+ Hits         1033     1035       +2     
  Misses        236      236              
- Partials       97       99       +2     

Flag	Coverage Δ
ubuntu-latest-unit	75.54% <35.71%> (-0.08%)	⬇️

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